A signalling system has been proposed for use in updating the prices being displayed on supermarket shelves. The system employs a number of LCD displays spaced along the supermarket shelf next to the goods to be sold. Each LCD display is controlled by a respective data tag having a microprocessor which communicates with a central computer system. The communication link between the microprocessor of each tag and the central computer system uses electromagnetic waves to carry the messages.
There are a number of problems with using electromagnetic waves as the medium for sending messages between the microprocessor and the tags. The most significant problem is that the use of electromagnetic waves in many frequency bands is restricted, leaving only high frequency bands for applications such as this. As a result, each of the tags much include a high frequency demodulator if it is to be able to receive transmitted messages from the central communication link. Additionally, if each tag is to be able to transmit messages back to the central computer system, then it will also require a high frequency modulation circuit. The requirement of having to use a demodulation circuit on its own or together with a modulation circuit, significantly increases the complexity and hence the cost of each of the tags.